mysqli：为IN ID中的每一个选择查询和限制

Question

我有contacts具有id | city_id | category_id |电子邮件字段的表。
基本的查询是

" 
SELECT id 
    , email 
    FROM contacts 
WHERE city_id IN (".implode(',', $arrCity).") 
    AND category_id IN (".implode(',', $arrCategory).") 
" 

但我需要设置一个限制：10排在城市的各个类别。可能吗？

Answer 1

这很棘手。在MySQL中，最简单的方法是使用变量：

SELECT id, email 
FROM (SELECT c.*, 
      (@rn := IF(@cc = CONCAT_WS(':', city_id, category_id), @rn + 1, 
         IF(@cc := CONCAT_WS(':', city_id, category_id), 1, 1) 
         ) 
      ) as rn 
     FROM (SELECT c.* 
      FROM contacts c 
      WHERE city_id IN (".implode(',', $arrCity).") AND 
        category_id IN (".implode(',', $arrCategory).") 
      ORDER BY city_id, category_id 
      ) c CROSS JOIN 
      (SELECT @cc := '', @rn := 0) params 
    ) c 
WHERE rn <= 10; 

这枚举了每个城市/类别组合中的行。外WHERE限制在每个组合10。